(a) Technical Field
The present invention relates to a fuel cell stack having a water drainage structure. More particularly, it relates to a fuel cell stack having a water drainage structure, which can effectively drain condensed water and prevent water from flowing into unit cells by combining an end anode plate (EAP) and an end cathode plate (ECP), which are formed by modifying an existing anode plate (AP) and cathode plate (CP) respectively into a dummy cell, and positioning the dummy cell at an end cell portion.
(b) Background Art
A fuel cell is an electricity generation system that electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack instead of converting chemical energy of fuel into heat by combustion. Notably, the reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. In certain embodiments, the fuel cell can be as an electric power supply source of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
Batteries and fuel cells are different in that in a fuel cell the reactant from an external source is consumed and must be replenished, i.e. it is a thermodynamically open system. In contrast, batteries, store electrical energy chemically and hence represent a thermodynamically closed system which does not consume a reactant from an external source.
Many combinations of fuels and oxidants are possible. For example, a hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. One of the most widely used fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which includes a fuel cell stack having a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member and one or more bipolar plates (BP). More specifically, the MEA includes a polymer electrolyte membrane through which hydrogen ions (i.e., protons) are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity and the gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member, on the other hand, functions to provide an appropriate bonding pressure. Finally, each bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
In particular, the fuel cell stack is composed of a plurality of unit cells, each of the unit cells including an anode, a cathode, and an electrolyte (electrolyte membrane). Hydrogen as fuel is supplied to the anode through a flow field of the anode plate (AP) and oxygen as oxidant is supplied to the cathode through a flow field of a cathode plate (CP). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through the GDL and the bipolar plate. At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen in the air supplied to the cathode to produce water. Migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
FIG. 5 shows an example of a typical fuel cell stack 1, and FIG. 6 shows an anode plate (AP) 10, one of bipolar plates (BP) used in the fuel cell stack 1.
The fuel cell stack 1 has a structure in which a plurality of unit cells 5 are connected to each other in series, in which air and hydrogen required to generate electricity are supplied to and discharged from one end of the fuel cell stack 1.
In detail, the air and hydrogen may be fed into an open end plate (EP) 3 with manifold apertures through a common distribution manifold 2, circulated through a power generation unit 7, and discharged through the open EP 3. Here, in a cathode loop, the air is supplied to the fuel cell stack 1 through a humidifier (not shown) and is then discharged to the outside of the vehicle through the humidifier. Moreover, in an anode loop, the hydrogen is supplied from a hydrogen tank (not shown) to the fuel cell stack 1 through a fuel processing system (FPS, not shown), and the residual hydrogen discharged from the fuel cell stack 1 is continuously circulated through the FPS and the fuel cell stack 1.
Each unit cell 5 of the power generation unit 7 has a structure in which the anode plate (AP), a first GDL, the MEA, a second GDL, and the cathode plate (CP) are sequentially stacked. As shown in FIG. 6, each of the anode plates (AP) 10 and the cathode plates (CP) comprises manifolds 12, 14, and 16 provided at both ends thereof, through which hydrogen, coolant, and air pass, respectively.
In the case of the anode plate (AP) 10, as shown in FIG. 6, hydrogen inlet/outlet apertures 13 connected to the hydrogen manifold 12 are formed such that the hydrogen can pass through a hydrogen flow field 15 formed on a reaction surface of the anode plate (AP) 10. A gasket 18 surrounding the outer edges of the anode plate (AP) 10 and the manifolds 12, 14, and 16 maintains the airtightness of fluids passing through the manifolds 12, 14, and 16 and the hydrogen flow field 15, and thus only the hydrogen passing through the hydrogen manifold 12 can be supplied to the hydrogen flow field 15 through the hydrogen inlet/outlet apertures 13. Moreover, in the case of the cathode plate (CP), air inlet/outlet apertures connected to the air manifold 16 are formed in the same manner, such that the air can pass through an air flow field formed on a reaction surface of the cathode plate (CP).
Meanwhile, during the above-described circulation process of the fuel cell stack, condensed water may be fed into a cathode inlet of the power generation unit through a humidifier, a common distribution manifold, an end plate, and the bipolar plate manifold. In doing so, the condensed water passes through the FPS, the common distribution manifold, the end plate, and the bipolar plate manifold. However, any water passing through the MEA may be fed from an anode inlet verses a cathode inlet.
In this case, the water flows into the outermost unit cells that are in contact with the open EP 3 to cause a rapid repetitive increase and decrease in cell voltage and a deterioration of MEA catalyst due to the presence of a large amount of water in the unit cells, which may specifically be a serious problem in the anode loop as it is a closed loop. Moreover, because the outermost unit cells adjacent to a close EP 4 have no manifold apertures, when the hydrogen or air is supplied through an inlet manifold of the bipolar plate, the water condensed in the manifold in the lengthwise direction of the fuel cell stack may flood into the closed EP 4 and thus be fed into the outermost unit cells.
Removal of the condensed water from the fuel cell stack other than the water required to humidify the MEA is a very important in terms of performance stability and durability of the fuel cell vehicle. Therefore, conventionally, a water trap, for example, may be provided to perform the water removal. However, this method does not remove the water as effectively as the industry would hope.
Moreover, U.S. Pat. No. 7,163,760 discloses a fuel cell stack having a bypass flow passage, in which a bypass plate and an intermediate plate are separately provided at an end portion (such as an end cell and an end plate) of a power generation unit of the fuel cell stack such that condensed water, which may flood into the power generation unit when hydrogen or air is supplied, is not fed into the power generation unit but discharged to the outside of the fuel cell stack. However, according to the above-described configuration, the bypass plate and the intermediate plate should be separately developed and added to the fuel cell stack, and thus the overall configuration of the fuel cell stack is further complicated when this method is utilized thereby indirectly increasing the overall costs of production. Thus, a cost efficient system and method for removal of the condensed water in a fuel cell stack is needed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.